This invention relates to a multi-layer information medium which has at least two data layers such as recording layer.
There is a growing need for an optical disk having a higher density and a higher capacity. DVD (Digital Versatile Disk) is already commercially available, and the DVD has a storage capacity of about 4.7 GB per single side which is about 7 times larger than the compact disk. Technologies enabling further increase in the amount of information recorded are under active development.
Technologies that have been used for increasing the recording capacity of an optical disk include use of a recording/reading beam having a short wavelength, use of an objective lens having a higher NA (numerical aperture) in the optical system irradiating the recording/reading beam, increase in the number of recording layers, and multi-value recording. Among these, three-dimensional recording by increasing the number of recording layers enables remarkable increase in the recording capacity at low cost compared to the use of shorter wavelength or use of a lens with a higher NA. The three dimensional recording medium is described, for example, in Japanese Patent Application Kokai (JP-A) 198709/1997, and JP-A 255374/1996 discloses a medium wherein a rewritable information storage layer and a read only information storage layer are laminated.
In the reading of a multi-layer recording medium including a plurality of recording layers by using an optical pickup which irradiates a reading beam, the optical pickup receives the beam reflected from the recording layer on which the reading beam had focused, and in addition, the beam reflected from the recording layer(s) other than the recording layer to which the reading beam had focused. This results in the signal interference between the plurality of recording layers, and cross talk is induced. As a consequence, noise is introduced in the read out signal. The influence of the beam reflected from the recording layer other than the target recording layer reduces inversely with the square of the distance between the recording layers. Therefore, increase in the distance between the adjacent recording layers is effective in reducing the noise induced. For example, when the medium is used with an optical pickup having the structure normally employed in DVD and other conventional optical disks, the recording layers are disposed at a mutual distance of at least 30xcexc, and preferably at least 70xcexc to realize the signal quality of practically acceptable level. This corresponds the Examples of the JP-A 198709/1997 wherein a transparent resin layer of 100xcexc thick is provided between the recording layers, and the JP-A 255374/1996 wherein two adjacent information storage layers are disposed at a distance of 30xcexc or more.
However, when the distance between the adjacent recording layers is increased to as large as 30xcexc or more, limitation in the number of recording layers in the medium will be required to avoid excessive increase in the disk thickness, and the total storage capacity of the disk will also be limited.
The transparent resin layer provided between the recording layers is also associated with a difficulty. To be more specific, formation of a transparent resin layer with a consistent thickness is difficult in spite of various attempts in forming the transparent resin layer by spin coating, resin sheet disposition and the like when the transparent resin layer formed is as thick as, for example, 30xcexc or more, and in particular, 70xcexc or more. The thick resin layer also suffers from increased internal stress and the medium is subject to warping. As a consequence, reliable provision with the optical disk of the required mechanical precision has been difficult.
In the case of the medium having a single recording layer formed on a substrate, the shape of the grooves (guide grooves) formed in the resin substrate will be transferred to the recording layer. In contrast, in the case of a medium wherein two or more recording layers are formed on the substrate with an intervening relatively thick transparent layer between the recording layers, it is quite difficult to transfer the shape of be grooves formed in the substrate to all of the recordig layers since the groove depth is about 100 nm at most for optical reasons while the distance between the recording layers is at least 30xcexc as described above. As a consequence, formation of the grooves in the transparent resin layer by photopolymerization (2P) process will be required as described, for example, in the JP-A 198709/1997 and eminent increase in the production cost is invited.
An object of the present invention is to realize a highly accurate tracking servo with no increase in the production cost in a multi-layer information medium wherein a plurality of data layers such as recording layers are disposed. Another object of the present invention is to suppress the cross talk associated with the decrease in the distance between the data layers.
Such objects are attained by the present invention. The invention comprises, in general terms, an optical information medium comprising at least two data layers for bearing recorded information, and a servo layer for bearing tracking servo information which is independently formed from the data layers; wherein the medium is used with a recording or reading system wherein a data beam for recording or reading the data in the data layer and a servo beam for reading the tracking servo information in the servo layer are used, and the servo layer is read by the servo beam that had passed through the data layer; and a filter layer is disposed between the data layer and the servo layer, and the filter layer exhibits higher absorption to the data beam than to the servo beam.
In one embodiment of the invention, the filter layer. exhibits an absorption of at least 80% to said data beam and an absorption of up to 20% to said servo beam. The filter layer may comprise a resin layer formed by UV curing a composition containing a UV-curable composition and a photoinitiator. The filter layer may contain a dye.
The optical information medium typically comprises at least two data layers for bearing recorded information, and a servo layer for bearing tracking servo information which is independently formed from said data layers, wherein the medium is used with a recording or reading system wherein a data beam for recording or reading the data in the data layer and a servo beam for reading the tracking servo information in the servo layer are used, and the servo layer is read by the servo beam that had passed through the data layer, and the servo layer exhibits lower reflectivity to the data beam than to servo beam.
The servo layer may comprise a metal or a semimetal. The data layer may be a flat layer, and the servo layer may be a layer formed with surface projections and depressions for bearing the tracking servo information. The data layer may be readable by using a confocal optical system.
In the present invention, the data layers are separately formed from the servo layer, and there is no need to form the tracking servo pattern on each data layer. Therefore, the data layer can be formed as a smooth layer, and highly accurate tracking servo is enabled with no increase in the production cost of the medium. Formation of the smooth data layer also results in the merits as described above. In the present invention, a filter layer is disposed between the data layer and the servo layer, and adverse effects on the tracking servo induced by the data beam reflected from the servo layer is thereby avoided.